1. Field of the Invention
This invention relates to an improved oxide bounding layer used to bond a thin hard wear resistant aluminum oxide coating to cutting tool inserts, and more particularly to a titanium monoxide layer used to join or bond a thin, hard, wear resistant, aluminum oxide coating to a cemented tungsten carbide substrate.
2. Description of Prior Art
Separate metal oxide layers between hard metal cutting tool inserts, such as cemented carbides, and ceramic coatings of aluminum oxide and the like have proven to be more effective bonding means when used directly between the ceramic coating and the adjacent cemented carbide substrate. Prior references to the use of a separate bonding layer including a metal oxide bonding layer are found in U.S. Pat. Nos. 3,736,107 Hale and 4,018,631 Hale, both of which are assigned to the same assignee as the present invention. In the 107 patent there is an intermediate bonding layer formed in situ and defined as a cobalt aluminate. In the 631 patent, a separate layer of a metal oxide, oxycarbide, or oxynitride, or titanium, for example, is disclosed as a bonding layer for aluminum oxide, Al.sub.2 0.sub.3.
A more desirable metal oxide bonding layer is a thin layer of titanium monoxide, TiO, for primary use directly between a cemented carbide substrate and a hard wear resistant ceramic coating such as Al.sub.2 O.sub.3. However, TiO layers are difficult to produce in a uniform cohesive and continuous layer with good bonding characteristics. One example of a use of TiO as a bond for other surfaces is disclosed in copending application Ser. No. 343,545 Graham filed Jan. 28, 1982 and assigned to the same assignee as the present invention. In this application TiO is used to bond a titanium nitride, TiN, coating directly to an Al.sub.2 O.sub.3 body or an Al.sub.2 O.sub.3 coating.